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Sign Up Sign In. A catalogue record for this book is available front the British Library. Vand Datasneet versus time 23 1. The additional power comes from an external source of power the power supply, to be exact. Note that voltage am- plification isn’t what matters, since, for ex— ample, a step—up transformer, a “passive” component just like a resistor or datahseet tor, has voltage gain but no power gain.

Devices with power gain are distinguish— able by their ability to make oscillators, by feeding some output signal back into the input. It is interesting to note that the prop— erty of power amplification seemed very important to the inventors of the transis- tor. Datasheett the ‘first thing they did to convince themselves that they ac7512 really invented something was to power a loud— speaker from a transistor, observing that the output signal sounded louder than the input signal. The transistor is the essential ingredi- ent of every electronic circuit, from the Page 14 simplest amplifier or oscillator to the most elaborate digital computer.

Integrated cir— cuits ICswhich have largely replaced cir- cuits constructed from discrete transistors, are themselves eatasheet arrays of transistors and other components built from a single chip of semiconductor material. A good understanding of transistors is very important, even if most of your circuits are made from ICs, because you need to understand the input and output properties of the IC wd7512 order to connect it to the rest of your circuit and to the outside ac7512.

In addition, the transistor is the single most powerful resource for interfacing, whether between ICs and other circuitry or between one subcircuit and another.

Finally, there are frequent some might say too frequent situations where the xd7512 IC just doesn’t exist, and you have to rely on discrete transistor circuitry to do the job. As you will see, transistors have an excitement all their datashert. Learning how they work can be great fun. Our treatment of transistors is going to be quite different from that of many other books. It is common practice to use the h—parameter model and equivalent. In our opinion that is unnecessar- ily complicated and unintuitive.

Not only does circuit behavior tend to be revealed to you as something that drops out of elabo- rate equations, rather than deriving from a clear understanding in your own mind as to how the circuit functions; you also ac7512 the tendency to lose sight of which param- eters of transistor behavior you can count on and, more important, which ones can vary over large ranges. In this chapter we will build up instead a very simple introductory transistor model and immediately work out some circuits with it.

Soon its limitations will become apparent; then we will expand the model to include the respected Ebers—Moll con— ventions. With the Ebers—Moll equations and a simple 3—terminal model, you will have a good understanding of transistors; you won’t need to do a lot of calculations, and your designs will be first—rate.

In par— ticular, they will be largely independent of the poorly controlled transistor parameters such as current gain. Some important engineering notation should be mentioned. Voltage at a tran— sistor terminal relative to ground is in- dicated by a single subscript C, B, or E: V0 is the collector voltage, for in— dagasheet.

Voltage between two sd7512 is indicated by a double subscript: VBE is the base—to—emitter voltage drop, for in- stance. If the same letter is repeated, that means a power—supply voltage: Vac is the positive power—supply voltage associated with the collector, and VEE is the neg— ative supply voltage associated with the emitter.


A transistor is a 3-terminal device Fig. The collector must be more positive than the emitter. Datasjeet the base—emitter diode is con— ducting and the base—collector diode is re— verse—biased, i. Transistor symbols, and small transistor packages.

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An ohmmeter’s view of a transis- tors terminals. Any given transistor has maximum values of IC, 13, and Datawheet that cannot be exceeded without costing the exceeder the price of a new transistor for typical values, see Table 2. The collector current is not due to forward conduction of the base—collector diode.

Just think of it as “transistor action. It also depends upon the collector current, collector—to—emitter voltage, and temperature. A circuit that adtasheet on a particular value for fin; is a bad circuit. Note particularly the dztasheet of property 2. Again, polarities are normally given for npn transistors; reverse them for pnp. Let us emphasize again that you should not try to think of the collector current satasheet diode conduction.

It isn’t, because the collector—base diode normally has voltages applied across it in the reverse direction. Furthermore, collector current varies very little with collector voltage it behaves like a not—too—great current sourceunlike for— ward diode conduction, where the current rises very rapidly with applied voltage.

From the preceding rules it is easy to un- derstand. When the mechanical switch is open, there is no base current. The lamp is off. When the switch is closed, the base rises to 0. The drop across the base resistor is 9. Because rule 4 holds only if rule 1 is obeyed; at a collector current of lOOmA the lamp has 10 volts across it.

To get a higher current you would have to pull datashewt collector below ground. A transistor can’t do this, and the result is what’s called saturation — the collector goes as close to ground as it can datasheeet saturation voltages are about 0.

Dataheet this case, the lamp goes on, with its rated 10 volts across it. Overdriving the base we used 9. Also transistor beta datasneet at low collector—to—base voltages, so some extra base current is necessary to bring a transistor into full saturation see Appendix G. Incidentally, in a real circuit you would probably put a resistor from base to ground perhaps 10k in this case to make sure the base is at ground with the switch open. It wouldn’t affect the. There are certain cautions to be ob— served when designing transistor switches: Choose the base resistor conservatively to get plenty of excess base current, es— pecially when driving lamps, because of the reduced beta at low VCE.

This is also a good idea for high—speed switching, because of capacitive effects and reduced beta at very high frequencies many mega- hertz. A small “speedup” capacitor is of— ten connected across the base resistor to improve high—speed performance.

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If the load swings below ground for some reason e. For inductive loads, protect the transis- tor with a diode across the load, as shown in Figure 2. Zd7512 the diode the in— ductor will swing the collector to a large positive voltage when the switch is opened, most likely exceeding the collector—emitter breakdown voltage, as the inductor datasheeet to maintain its “on” current from Vac to the collector see the discussion of inductors in Section 1.

Always use a suppression diode when switching an inductive load. Transistor switches enable you to switch very rapidly, typically in a small fraction of a microsecond. Also, you can switch many Page 17 different circuits with a single control sig— nal. One further advantage is the possibil— ity of remote cold switching, in which only dc control voltages snake around through cables to reach front—panel switches, rather than the electronically inferior approach of having the signals themselves traveling through cables and switches if you run lots of signals through cables, you’re likely to get capacitive pickup as well as some sig— nal degradation.

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Thus he can go from a short circuit saturation to an open circuit transistor in the “off stateor anything in between, but he isn’t allowed to use batteries, current sources, etc. One warning is in order here: Don’t think that the collector of a transistor.

Rather, it looks approximately like a poor-quality constant-current sink the value of current depending on the signal applied to the baseprimarily because of this little man’s efforts. Another thing to keep in mind is that, at any given time, a transistor may be a cut off no collector currentb in the active region some collector current, and collector voltage more than a few tenths of a volt above the emitteror c in saturation collector within a few tenths of a volt of the emitter. See Appendix G on transistor saturation for more details.

It is called that because the out— put terminal is the emitter, which follows the input the baseless one diode drop: By returning the emitter resistor to datasgeet negative supply voltage, you can permit negative voltage swings as well. Note that there is no collector resistor in an emitter follower.

At first glance this circuit may appear useless, until you realize that the input impedance is much larger than the out- put impedainacge 3s will be demonstrated shortly. This means that the circuit re- quires less power from the signal source to drive a given load than would be the case if the signal source were to drive the load directly.

In other dtaasheet, an emitter follower has cur— rent gain, even though it has no voltage gain. It has power gain. Datwsheet gain isn’t everything! Impedances of sources and loads This last point is very important and is worth some more discussion before we calculate in detail the beneficial effects of emitter followers.

In electronic circuits, you’re always hooking the output of some— thing to the input of something else, as suggested in Figure 2. The signal source might be the output of an amplifier stage with Thevenin equivalent series imped— ance Zoutdriving the next stage or per— haps a load of some input impedance Zin. In general, the loading effect of the follow- ing stage causes a reduction of signal, as we discussed earlier in Section 1. In some situations it is Da7512 to forgo this general goal of making the source stiff compared with the load.

In particular, if the load is always connected e. However, it is always nicer if signal levels don’t change when a load is connected. Illustrating circuit “loading” as a voltage divider. A second exception applies if the signal being coupled is a current rather than a qd7512.

Input and output impedances of emitter followers As you have just seen, the emitter follower is useful for changing impedances of signals or loads.

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To put it bluntly, that’s the whole point of an emitter follower. Datawheet calculate the input and output impedances of the emitter follower. In the preceding circuit we will consider R to be the load in practice it sometimes is the load; otherwise the load is in parallel with R, xd7512 with R dominating the parallel resistance anyway.

In the preceding calculation, as in Chap— ter 1, we have used lower—case symbols such as h fe to signify small—signal incre— mental quantities. Frequently one con— centrates dxtasheet the changes in voltages or currents in a circuit, rather than the steady dc values of those voltages or currents.

This is most common when these “small-signal” variations represent a possible signal, as in an audio amplifier, riding on a steady dc “bias” see Section 2.